An Investigation on Prediction of Stretch Edge Cracking in Advanced High Strength Steels

Abstract

Advanced High Strength Steels (AHSS) are increasingly being used in the automotive industry for vehicle body manufacturing to reduce overall weight and increase fuel efficiency. Major challenges associated with forming AHSS sheets arise due to edge cracking of blanked/trimmed edges that occurs at much lower strains than those predicted by traditional Forming Limit Diagrams (FLDs). In this work a comprehensive experimental and numerical investigation was executed to understand the fracture behavior of AHSS and establish Finite Element Analysis (FEA) model to predict limiting strains. Experimentation involved observation of the effects of sample parameters of single notched samples such as notch radius, material orientation and edge quality, on fracture strains of various AHSS grades. Execution of a general numerical modeling study helped validate and establish anisotropic material modelling technique in ABAQUS code for DP 980 BARE and CR 780 grades. Limiting strains were numerically identified using second time derivative of thickness strain method and compared to experimental results obtained using Digital Image Correlation (DIC) strain analysis.

Edge quality played a critical role in determining formability property of samples while considering various notch radii. The smooth-edge samples showed greater formability with smaller notch radius (higher strain gradient) which contrasted with sheared-edge samples that displayed lower formability for smaller notch radius. Additionally, FEA predicted limiting strains were lower than actual fracture strains for smooth-edge while they were slightly higher for sheared-edge samples. A new Failure Strain Factor was introduced as empirical relation between FEA predicted limiting strains and DIC fracture strains for sheared-edge samples. This testing and FEA prediction method provided conservative results for smooth-edge and favorable results for sheared-edge samples with the use of Failure Strain Factor. Consequently, it was concluded that this method could be useful in metal forming industry for predicting limiting strains for AHSS.